Paleomagnetic evidence for crustal and thin‐skinned rotations in the European Hercynides

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94849/1/grl3086.pd

Paleomagnetism and the orocline hypothesis

Oroclines were originally defined by Carey as curved mountain belts which initially were straight, or at least straighter than they are today. In the last few years, the definition has been broadened to include any curved mountain belt, regardless of its original shape.Since the occurrence of oroclinal bending is best recorded in the change of declination as a function of tectonic setting, paleomagnetic and structural data from six potential oroclines have been compiled and analyzed to determine the amount of rotation displayed by the change of paleomagnetic declination relative to the change in strike of the fold belt.The arcuate belts investigated are: the Sicilian-Calabrian Arc and the Umbrian Arc of Italy, the Swiss portion of the Jura Mountains, the central portion of the Appalachian Mountains (from Pennsylvania to Virginia, U.S.A.), the Wyoming-Idaho overthrust belt of western North America and the Hercynides of Western and Central Europe.The Jura Mountains and the Pennsylvania-Virginia portion of the Appalachians fail to show significant oroclinal bending. The Wyoming-Idaho belt shows a combination of rotated (possibly oroclinal) and unrotated thrust sheets.In the Sicilian-Calabrian Arc significant oroclinal bending caused by the impingement of the Calabria-Peloritani nappes in the Late Tertiary can be demonstrated, while the Umbrian Arc of similar age, in the Northern Apennines, also shows oroclinal bending on a smaller scale.Hercynian Europe (the only belt included in which deformation of basement rocks can be demonstrated) shows oroclinal bending (at least 80[deg]) as well as a marked original curvature (70[deg]) in its western part.Common to all the oroclines studied in this paper is the probable impingement of a rigid block or continental margin during the orogeny, causing subsequent deformation and bending of the fold belt.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/25531/1/0000072.pd

Paleomagnetism of the western Cape Fold belt, South Africa, and its bearing on the Paleozoic apparent polar wander path for Gondwana

In order to test two different proposals for the poorly defined African Paleozoic apparent polar wander path (APWP), a paleomagnetic study was carried out on Ordovician through Carboniferous clastic sediments from the Cape Fold belt, west of the 22nd meridian. One proposal involves a relatively simple APWP connecting the Ordovician Gondwana poles in North Africa with the Late Paleozoic poles to the east of South Africa in a more or less straight line crossing the present equator in the Devonian. The other proposal adds a loop to this path, connecting Ordovician poles in North Africa with poles to the southwest of South Africa and then returning to central Africa. This loop would occur mainly in Silurian time. New results reported herein yield paleopoles in northern and central Africa for Ordovician to lowermost Silurian and Lower to Middle Devonian formations. The best determined paleopole of our study is for the Early Ordovician Graafwater Formation and falls at 28[deg]N, 14[deg]E (k = 25, [alpha]95 = 8.8[deg], N = 28 samples). The other paleopoles are not based on sufficient numbers of samples, but can help to constrain the apparent polar wander path for Gondwana. Our results give only paleopoles well to the north of South Africa and we observe no directions within the proposed loop. Hence, if the loop is real, it must have been of relatively short duration (60-70 Ma) and be essentially of Silurian/Early Devonian age, implying very high drift velocities for Gondwana (with respect to the pole) during that interval.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26629/1/0000170.pd

Chemical composition, age, and magnetic parameters of igneous rocks from the Franz Josef Land

A combined study of magnetic parameters of basalt and andesite samples has been carried out in the framework of geological investigations of the Franz Josef Land. This study has included determination of coercivity, saturation magnetization, Curie points, natural remanent magnetization (NRM), and magnetic susceptibility as well as examination of ferromagnetic minerals with a microscope. Data on chemical composition of the rocks have been obtained for all the samples, and radiological ages have been determined for the majority of the rocks. Thermomagnetic curves of the samples have been subdivided into four types depending on composition of ferromagnetic NRM carriers. Data showing multiple changes in the predominant composition of the igneous rocks have been obtained. Each stage of magmatism is characterized by a specific type of the ferromagnetic component in the rocks and, therefore, magnetomineralogical investigations can be used for differentiation and correlation of the igneous rocks